Packers are used downhole to isolate portions of a wellbore from each other. There are many styles of packers. Some set by longitudinal compression of the sealing element by fluid pressure applied to a setting tool or by mechanical force such as from setting down weight. Other designs involve elements that are inflated. More recently, elements that swell to a sealing position on exposure to well fluids have been used. There have been many variations as outlined below.
Packers have been used that employ elements that respond to the surrounding well fluids and swell to form a seal. Many different materials have been disclosed as capable of having this feature and some designs have gone further to prevent swelling until the packer is close to the position where it will be set. These designs were still limited to the amount of swelling from the sealing element as far as the developed contact pressure against the surrounding tubular or wellbore. The amount of contact pressure is a factor in the ability to control the level of differential pressure. In some designs there were also issues of extrusion of the sealing element in a longitudinal direction as it swelled radially but no solutions were offered. A fairly comprehensive summation of the swelling packer art appears below:
I. References Showing a Removable Cover Over a Swelling Sleeve
1) Application US 2004/0055760 A1                FIG. 2a shows a wrapping 110 over a swelling material 102. Paragraph 20 reveals the material 110 can be removed mechanically by cutting or chemically by dissolving or by using heat, time or stress or other ways known in the art. Barrier 110 is described in paragraph 21 as an isolation material until activation of the underlying material is desired. Mechanical expansion of the underlying pipe is also contemplated in a variety of techniques described in paragraph 24.        
2) Application US 2004/0194971 A1                This reference discusses in paragraph 49 the use of water or alkali soluble polymeric covering so that the actuating agent can contact the elastomeric material lying below for the purpose of delaying swelling. One way to accomplish the delay is to require injection into the well of the material that will remove the covering. The delay in swelling gives time to position the tubular where needed before it is expanded. Multiple bands of swelling material are illustrated with the uppermost and lowermost acting as extrusion barriers.        
3) Application US 2004/0118572 A1                In paragraph 37 of this reference it states that the protective layer 145 avoids premature swelling before the downhole destination is reached. The cover does not swell substantially when contacted by the activating agent but it is strong enough to resist tears or damage on delivery to the downhole location. When the downhole location is reached, pipe expansion breaks the covering 145 to expose swelling elastomers 140 to the activating agent. The protective layer can be Mylar or plastic.        
4) U.S. Pat. No. 4,862,967                Here the packing element is an elastomer that is wrapped with an imperforate cover. The coating retards swelling until the packing element is actuated at which point the cover is “disrupted” and swelling of the underlying seal can begin in earnest, as reported in Column 7.        
5) U.S. Pat. No. 6,854,522                This patent has many embodiments. The one in FIG. 26 is foam that is retained for run in and when the proper depth is reached expansion of the tubular breaks the retainer 272 to allow the foam to swell to its original dimension.        
6) Application US 2004/0020662 A1                A permeable outer layer 10 covers the swelling layer 12 and has a higher resistance to swelling than the core swelling layer 12. Specific material choices are given in paragraphs 17 and 19. What happens to the cover 10 during swelling is not made clear but it presumably tears and fragments of it remain in the vicinity of the swelling seal.        
7) U.S. Pat. No. 3,918,523                The swelling element is covered in treated burlap to delay swelling until the desired wellbore location is reached. The coating then dissolves of the burlap allowing fluid to go through the burlap to get to the swelling element 24 which expands and bursts the cover 20, as reported in the top of Column 8)        
8) U.S. Pat. No. 4,612,985                A seal stack to be inserted in a seal bore of a downhole tool is covered by a sleeve shearably mounted to a mandrel. The sleeve is stopped ahead of the seal bore as the seal first become unconstrained just as they are advanced into the seal bore.II. References Showing a Swelling Material Under an Impervious Sleeve        
1) Application US 2005/0110217                An inflatable packer is filled with material that swells when a swelling agent is introduced to it.        
2) U.S. Pat. No. 6,073,692                A packer has a fluted mandrel and is covered by a sealing element. Hardening ingredients are kept apart from each other for run in. Thereafter, the mandrel is expanded to a circular cross section and the ingredients below the outer sleeve mix and harden. Swelling does not necessarily result.        
3) U.S. Pat. No. 6,834,725                FIG. 3b shows a swelling component 230 under a sealing element 220 so that upon tubular expansion with swage 175 the plugs 210 are knocked off allowing activating fluid to reach the swelling material 230 under the cover of the sealing material 220.        
4) U.S. Pat. No. 5,048,605                A water expandable material is wrapped in overlapping Kevlar sheets. Expansion from below partially unravels the Kevlar until it contacts the borehole wall.        
5) U.S. Pat. No. 5,195,583                Clay is covered in rubber and a passage leading from the annular space allows well fluid behind the rubber to let the clay swell under the rubber.        
6) Japan Application 07-334115                Water is stored adjacent a swelling material and is allowed to intermingle with the swelling material under a sheath 16.III. References Which Show an Exposed Sealing Element that Swells on Insertion        
1) U.S. Pat. No. 6,848,505                An exposed rubber sleeve swells when introduced downhole. The tubing or casing can also be expanded with a swage.        
2) PCT Application WO 2004/018836 A1                A porous sleeve over a perforated pipe swells when introduced to well fluids. The base pipe is expanded downhole.        
3) U.S. Pat. No. 4,137,970                A swelling material 16 around a pipe is introduced into the wellbore and swells to seal the wellbore.        
4) U.S. Application US 2004/0261990                Alternating exposed rings that respond to water or well fluids are provided for zone isolation regardless of whether the well is on production or is producing water.        
5) Japan Application 03-166,459                A sandwich of slower swelling rings surrounds a faster swelling ring. The slower swelling ring swells in hours while the surrounding faster swelling rings do so in minutes.        
6) Japan Application 10-235,996                Sequential swelling from rings below to rings above trapping water in between appears to be what happens from a hard to read literal English translation from Japanese.        
7) U.S. Pat. Nos. 4,919,989 and 4,936,386                Bentonite clay rings are dropped downhole and swell to seal the annular space, in these two related patents.        
8) US Application US 2005/0092363 A1                Base pipe openings are plugged with a material that disintegrates under exposure to well fluids and temperatures and produces a product that removes filter cake from the screen.        
9) U.S. Pat. No. 6,854,522                FIG. 10 of this patent has two materials that are allowed to mix because of tubular expansion between sealing elements that contain the combined chemicals until they set up.        
10) U.S. Application US 2005/0067170 A1                Shape memory foam is configured small for a run in dimension and then run in and allowed to assume its former shape using a temperature stimulus.        
Swelling materials such as foams are generally porous structures so that even when they swell or are released from a confined position and allowed to take on well fluids and resume a relaxed position are still limited in their sealing ability. With the passages in the foam still open even after swelling, increases in differential pressure can still reshape the element and perhaps cause leakage past it. What is needed is a swelling element that has the capability of obstructing or even sealing off passages within it in conjunction with swelling so that the resulting swollen structure is less porous or even impervious and that forms a more enduring seal. The present invention provides this structure and other features to enhance the sealing capability of downhole devices. The invention will be more readily understood by those skilled in the art from a review of the description of the preferred embodiment and the associated drawings while the appended claims below define the full scope of the invention.